1. Field of the invention
This invention relates to a slowly-breaking fuse suited for protecting an electric motor such as a power window motor of an automobile.
2. Related Art
Generally, the type of fusion of fuses used for protecting an electric circuit in an automobile or the like is classified into the fusion in a high current region and the fusion in a low current region. One example of the former fusion in a high current region is a fusion due to a burst current developing in the event of a dead short-circuit of a circuit, and in this case the time period from heat generation to fusion is relatively short, that is, within several seconds.
On the other hand, in the latter fusion in a low current region, the temperature of the fuse gradually rises as when a low excess current is lasting for a long period of time, and then the fuse is fused. Thus, it takes a relatively long period of time from heat generation to the fusion of the fuse.
For example, when a load circuit of an electric motor or the like is activated, an instantaneous excess current, several times larger than a steady-state load current value, flows through the circuit. When a power window motor is in a motor lock condition upon full opening or closing of a window glass pane, a motor lock current, several times larger than a steady-state load current value, flows through it, and therefore even when any abnormal condition, such as short-circuit of the circuit is not encountered, the current, exceeding the steady-state current value, frequently flows.
Therefore, in a load circuit for an electric motor, a power window motor and so on, there has been used a so-called slowly-breaking fuse of gentle characteristics which will not fuse for such instantaneous excess current and motor lock current exceeding a steady-state current value, but will positively interrupt the excess current in the event of slight short-circuit. In the slowly-breaking fuse, a piece of low-melting metal, having high thermal conductivity and a good heat-absorbing property, is supported on a generally central portion of the fuse of high-melting, fusible metal, and the heat, generated in a melting portion by excess current during use, is transferred to and absorbed by the low-melting metal piece, thereby ensuring a time lag before fusion occurs. Namely, an allowable range of the melting portion is increased by the low-melting metal piece so that even if the excess current flows, the melting portion will not instantaneously be fused, thereby ensuring the retarded fusion.
At this time, if this retarded breakage becomes too excessive, a wire and a housing become heated, in which case a covering of the wire may be burned, or the housing may melt. Therefore, the slowly-breaking fuse must be fused without delay when the predetermined time lag is over.
By the way, recently, a fuse has been required to have a small design and an increased rated current value, and it has become necessary to specify the position of the melting portion so that the housing will not be burned by the heat of the fuse, and also it has become necessary to narrow the space of the heating portion in order to suppress the transfer of the heat to fuse terminals. Therefore, if there is provided a construction by which the position of development of the above hot spot (the melting portion) is fixed, and the hot spot is reduced as much as possible so as to effect the fusion at an extremely narrow region, then the transfer of the heat to the vicinity portions including the wire and the fuse terminals can be reduced, thereby suppressing the overall heating of the fuse, and besides the heating of the fuse is effectively used, thereby achieving the slowly-breaking fuse excellent in fusion sensitivity even in a low current region.
Therefore, in order to specify the position of the melting portion and also to narrow the space of the heating portion, it is necessary to provide such a construction that the current density at the hot spot becomes extremely high as compared with the current density of the vicinity portions, and also it is necessary to provide such a construction that the current density at the hot spot is abruptly increased with respect to the current density of the vicinity portions. Such current density characteristics can be achieved by extremely reducing the cross-sectional area of that portion of the fuse element at which the hot spot develops, with respect to the cross-sectional area of those portions disposed forwardly and rearwardly adjacent to this hot spot portion. The higher the constriction rate of the hot spot is, the more easily the fusion can be effected at the above extremely narrow region.
There have been proposed the following various methods of improving the fusion sensitivity by increasing the constriction rate of the hot spot.
For example, as shown in FIGS. 5 (a) and (b), a fuse 100 of the plug-in type disclosed in Japanese Patent Unexamined Publication sho. 50-101845 comprises as a constituent part a fuse element 102 inserted into a housing 101, and this fuse element 102 has a pair of blade-like terminals 103 and 103, and a link portion 105 interconnecting these terminals. The link portion 105 is cut into a thin configuration by milling, and after this cutting operation, this link portion is compressed to form a melting portion at a generally central portion thereof.
As shown in FIG. 5(b) which is a cross-sectional view, the thickness t.sub.0 of the generally central portion of the link portion 105 serving as the melting portion is made smaller than the thickness t.sub.1 and t.sub.2 of those portions disposed forwardly and rearwardly adjacent to this melting portion, so that the cross-sectional area S.sub.0 of the melting portion is smaller than the cross-sectional area S.sub.1 and S.sub.2 of the forwardly and rearwardly adjacent portions. By doing so, the constriction rate of the hot spot is increased, thereby improving the fusion sensitivity.
In a fuse shown in FIGS. 6 (a) and (b) which is disclosed in U.S. Pat. No. 4,831,353, a fuse element 120 formed by stamping a metal plate has a link portion 121 on which a plurality of weak spots 122, 123 and 124 are formed.
As shown in FIG. 6(b) which is an enlarged view of an important portion, the cross-sectional area S.sub.0 of each weak spot is made smaller than the cross-sectional area S.sub.1 and S.sub.2 of the forwardly and rearwardly adjacent portions, so that the constriction rate of the hot spot is increased, thereby improving the fusion sensitivity.
In a fuse shown in FIG. 7 which is disclosed in Japanese Patent Unexamined Publication sho. 61-271731, a fuse element 131 inserted into a housing 130 has an S-shaped link portion 132 in which a notch 133 defined by a through hole is formed. Thus, the cross-sectional area is minimized at the position where this notch 133 is provided, thereby increasing the constriction rate of the hot spot so as to improve the fusion sensitivity.
However, the above conventional slowly-breaking fuses have problems. More specifically, the fuse disclosed in the above Japanese Patent Unexamined Publication sho. 50-101845, in which the thickness of the link portion is reduced by milling and compressing, suffers from a drawback that the machining costs are extremely increased for achieving a strict tolerance. The fuses disclosed in the above U.S. Pat. No. 4,831,353 and the above Japanese Patent Unexamined Publication sho. 61-271731 have problems that an expensive die is required for forming the link portion of a complicated shape and that it is difficult to increase the yield rate of satisfactory products, and thus the problem with respect to the increased processing cost remains unsolved.
To increase the constriction rate of the hot spot by such machining is limited, and the constriction rate could not be increased any further. Namely, generally, in the stamping by a press, a processing limit is the width about twice larger than the plate thickness, and the fuse having the constricted portion of a smaller width than that value can not be formed by pressing.